1. Field of the Invention
This invention relates to sensors having relatively movable capacitor plates.
2. Prior Art
It is known to have an acceleration sensor using a movable plate that moves rotationally above a fixed plate about a torsion spring. Because of the torsional nature of the spring, under accelerations, one side of the movable plate is moved further away from the fixed plate while the other side of the movable plate moves closer to the fixed plate. By applying a voltage between a movable plate and the metal on the fixed plate, the separation distance between the plates can be determined by a capacitance measurement.
These two capacitances, measured as CA and CB, are converted into an electrical signal. In this embodiment, mechanical to capacitance transduction is defined by DelC=(CA-CB)/(CA+CB). The forces generated by the voltages used to measure the separation distance between the plates perturbs the system significantly. The result being that the final steady-state position of the moveable plate is determined by the value of the applied acceleration and the applied sensing voltages. Since the voltage force is proportional to the inverse of the plate separation squared, (1/separation).sup.2, the size of the voltage effect is a nonconstant and nonlinear function of the applied g force. This nonconstant force in turn causes the expression of DelC to be nonlinear with respect to acceleration.
Micro-miniature, solid-state accelerometers are used for a number of important applications such as acceleration sensors in missile safe and arm devices. One prior solid-state accelerometer comprises a mass supported by a silicon beam upon which one or more piezoresistive sensing elements are formed. Under acceleration, the restoring force exerted by the beam on the mass induces stress in the sensing element. The resistance of the sensing element changes with the stress, and the change in resistance is converted to a differential voltage by using one or two sensing elements in a resistance bridge circuit.
Other known prior art includes U.S. Pat. Nos. 4,736,629 (Cole), and 5,028,876 (Cadwell).
It would be desirable to optimize the linearity of such a device without changing other functionality of the device. Known methods of changing linearity of the device have some significant effect on other functional parameters of the design. These are some of the problems this invention overcomes.